In a therapy radiation therapy system with high accuracy, for example, an X-ray radiation system with high accuracy or a particle therapy system, a known one is one using X-ray irradiation for accurately specifying a position of a target volume.
When X-ray photography is performed for a subject, to avoid unnecessary X-ray exposure or deterioration in image quality, an X-ray diaphragm is provided between an X-ray tube and an X-ray detection unit, and a controller controlling the position of each blade of the X-ray diaphragm is operated, thereby setting the X-ray irradiation range.
In the conventional X-ray diaphragm, there is provided a potentiometer outputting a voltage in accordance with displacement of a drive motor to the drive motor connected to each blade. Thus, before X-ray imaging, an opening state of the X-ray diaphragm does not need to be in an initial state, and a target position of the blade is set in relation to the current position of the blade which has been detected by the potentiometer, thereby setting the X-ray irradiation range. This technique is disclosed in Patent Literature 1.